CN107850651B

CN107850651B - RF transmit module with local field monitoring unit for magnetic resonance examination system

Info

Publication number: CN107850651B
Application number: CN201680041443.2A
Authority: CN
Inventors: C·洛斯勒; P·韦尔尼科尔
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2015-07-15
Filing date: 2016-07-12
Publication date: 2021-01-26
Anticipated expiration: 2036-07-12
Also published as: RU2018100990A; EP3322996B1; WO2017009315A1; RU2018100990A3; US10495705B2; JP2018524111A; EP3322996A1; US20190079151A1; RU2713807C2; JP6857646B2; CN107850651A

Abstract

A Radio Frequency (RF) transmit module for a magnetic resonance examination system is disclosed. The local field monitoring unit measures the field emitted by the RF emitting element and generates a puc signal. The puc signal is amplified and down-converted by mixing with the oscillator signal. The down-converted puc signal and the RF drive signal for the RF transmitting element are transferred through the common signal lead. The oscillator signal may also be transferred via a common signal lead.

Description

RF transmit module with local field monitoring unit for magnetic resonance examination system

Technical Field

The invention relates to a Radio Frequency (RF) transmit module with a local field monitoring unit.

Background

Such an RF transmit module is known from International application WO 2006/114749. The known RF transmit module is formed by a circuit arrangement for operating a multi-channel transmit/receive antenna in a magnetic resonance examination system.

The known circuit arrangement comprises a plurality of RF coils (coil segments) each connected to a transmit/receive channel. A multi-channel RF amplifier (or several single-channel RF amplifiers) is coupled to the RF coil through the transmit/receive channels via a transmit/receive switch. Furthermore, a plurality of pick-up coils is provided for receiving RF signals for monitoring purposes. The RF signals received by these pick-up coils are routed via the transmit/receive switch to the pick-up coil detection unit for processing. The pick-up coil is connected to the pick-up coil detection unit through a separate channel.

Disclosure of Invention

It is an object of the invention to provide an RF transmit module for a magnetic resonance examination system which has a simpler setup.

This object is achieved according to the invention by an RF transmit module comprising:

-an RF transmitting antenna element for transmitting RF signals,

an RF power supply with an RF control to generate an RF drive signal,

a signal lead electrically connected between an RF power source and an RF transmitting antenna element for supplying the RF driving signal to the RF transmitting element,

a local field monitoring unit for measuring the local field strength transmitted by the RF transmit antenna elements and generating a puc signal representing the measured local field strength,

-a mixer configured to frequency convert the puc signals into frequency converted puc signals and electrically connected between the local field monitoring unit and the signal lead to transmit the frequency converted puc signals to the RF control through the signal lead. The RF transmit module of the invention comprises RF transmit elements, for example formed by coil conductor loops, loops or rungs of a birdcage or TEM coil, dipoles or dielectric resonators. The RF transmitting element is driven from an RF power source by an RF drive signal transmitted over the signal lead. The signal lead may be a coaxial cable. A local field monitoring unit, such as a pick-up coil, a local current sensor or a local capacitor, is used to measure the local field strength of the RF transmit element. The puc signal is frequency converted (down or up) and transmitted to the RF control via the signal leads. The RF control controls the RF power supply. The RF power supply may include an RF power amplifier controlled by an RF control. The puc signal typically has a bandwidth of about 700kHz and can be (down-) converted to a frequency-converted puc signal having a carrier frequency in a range between 1MHz and 100MHz, or between 5MHz and 100MHz, or between 10MHz and 100 MHz. Good results are obtained by (down) frequency conversion to a frequency band at 50 MHz. Within these ranges, a narrow band around the larmor frequency (42 MHz/T of protons) at the tissue is to be excluded to avoid interference with the radio frequency operation of the transmission by the RF antenna and the acquisition of the magnetic resonance signals. Typically, the signal leads are formed from coaxial cables having low loss in the frequency range of 1, 5 or 10MHz to 100 MHz.

In one aspect of the invention, the RF transmit module enables transmitting the measured local field strength represented by the frequency converted puc signal and the RF drive signal on the same signal line. In this way, hardly any additional wiring is needed to transfer the measured local field strength to the control of the RF power supply. The frequency conversion is preferably performed downwards. This results in less loss than a single signal lead. Preferably, the frequency may be selected according to commercially available components (e.g., filters). Alternatively, the mixer may be configured to mix down to the zero frequency of the frequency converted puc signal. That is, the puc signal is directly converted into a DC puc signal. This can be achieved with an inexpensive ADC and simpler filter components.

In summary, the Radio Frequency (RF) transmission module of the present invention comprises: a local field monitoring unit that measures the field emitted by the RF emitting element and generates a puc signal. The puc signal is amplified and down-converted by mixing with the oscillator signal. The signal leads serve as common signal leads for the RF drive signal and the puc signal. The down-converted puc signal and the RF drive signal for the RF transmitting element are transferred through the common signal lead. The oscillator signal may also be transferred through a common signal line. Based on the puc signal, B generated by the RF transmit element can be monitored₁A field. For example, the puc signal may represent local RF current in the coil loop. Furthermore, the puc signal is the basis for SAR control and monitoring and system logging. In addition, the puc signal can be used as an input to an RF power supply feeding the RF transmitting element, in particular to perform digital predistortion on the RF power supply.

The local field monitor unit may comprise local pick-up coils to pick-up the flux of the field from the RF transmit elements. Other types of field sensors may also be employed which are sensitive to the local field strength of the RF transmit element and form a puc signal indicative of the local field strength. For example, an electric dipole antenna sensitive to the electric field component of the field may be employed to measure the electric field component and generate a puc signal at the RF transmitting element representative of the electric field strength. Further, a capacitive voltage divider may be employed to generate the puc signal. Alternatively, the local field monitoring unit may be implemented as a voltage or current sensor to measure the voltage/current of the forward and reflected power to and from the RF transmit element and to derive the local field strength from the measured voltage/current. These can be measured at the directional coupler between the RF power supply and the RF transmit element.

These and other aspects of the invention will be further elaborated with reference to the embodiments defined in the dependent claims.

In an embodiment of the invention, the preamplifier is electrically connected between the local field monitoring unit and the mixer. Preferably, the local field monitoring unit draws only a small amount of power from the field emitted by the RF emitting element. In this way, the local field monitoring has only a small effect on the power efficiency of the RF transmit element. The low signal picked up by the local field monitoring unit is amplified by a preamplifier. Preferably, the preamplifier is located close to the local field monitoring unit. This avoids that the electrical conductor between the local field monitoring unit and the preamplifier can pick up signals from the signal leads and thereby cause cross-talk between the local field monitoring unit and the signal leads. In this way, crosstalk is reduced with the other antenna elements (coil conductors of the other coils). The preamplifier and the local field monitoring unit are preferably integrated in a single unit. This further enables the local field monitoring unit to be located close to the RF transmit element, in particular if the local field monitoring unit is a pick-up coil, wherein the conductor loop is magnetically sensitive. If the local field monitoring unit is sensitive to electric fields, for example forming a local field monitoring unit, a dipole or a small dielectric resonator is positioned near the tuning capacitor of the RF transmitting element. Preamplifiers typically have a gain of 25dB and therefore the frequency converted puc signal can be transmitted through the signal leads with a loss of 5dB to 10 dB.

In further embodiments, the invention is used in a multi-channel RF transmit module. In this embodiment there are a plurality of RF transmission elements, which are fed with RF drive signals through the signal leads of each channel. Several local field monitoring units are provided to measure the local field strength of the RF transmitting elements. The puc signals of these local field monitoring units are frequency-converted by the mixer and it is ensured that the frequency-converted puc signals are emitted through the signal leads of the respective channels. The mixer may be implemented as a mixer unit for the respective channels, or the mixer unit may be shared by a group of channels.

In a further embodiment of the invention, the RF transmit module is provided with matching circuitry for impedance matching between the RF transmit antenna elements and the signal leads. A mixer notch filter is provided to attenuate the RF drive signal so as to avoid the RF drive signal from reaching the mixer.

In a further embodiment of the invention, the oscillator signal of the mixer is also supplied via the signal lead. To this end, an oscillator for generating an oscillator signal is coupled into the signal lead. An oscillator notch filter is electrically connected between the oscillator and the signal lead to attenuate the RF drive signal and prevent the RF drive signal from reaching the oscillator. The oscillator signal is supplied from the signal leads to the mixer via the mixer notch filter. The mixer notch filter and the oscillator notch filter isolate the mixer and the oscillator, respectively, from the high power RF drive signal. Preferably, the mixer-duplexer is electrically connected between the mixer notch filter and the mixer to apply the oscillator signal to the mixer. The mixer-duplexer separates the oscillator signal to the mixer from the down-converted puc signal from the mixer. The mixer generates a frequency converter puc signal based on the applied oscillator signal and the amplified puc signal. Preferably, an oscillator-duplexer is electrically connected between the oscillator notch filter and the oscillator to feed the oscillator signal to the signal leads. The oscillator-duplexer separates the oscillator signal from the oscillator from the down-converted puc signal coupled out from signal lead 111.

In further embodiments, the matching circuit, the mixer notch filter, the mixer-duplexer and local pick-up unit and the preamplifier are integrated into a single module, e.g., on a single printed circuit board. The integrated module is readily coupled to the signal leads and the RF antenna element.

Alternatively, the hybrid-diplexer and hybrid can be remotely located from the local field monitoring unit (e.g., typically in a 10-20cm coil housing).

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter and with reference to the accompanying drawings.

Drawings

FIG. 1 is a schematic representation of an RF transmit module of the present invention;

FIG. 2 is a schematic representation of a more detailed embodiment of the RF transmit module of the present invention;

figures 3 and 4 show diagrams of matching circuits employed in the present invention;

figures 5, 6 and 7 show diagrams of alternatives to stop band notch filters.

Fig. 8 shows a circuit diagram of an example of a duplexer;

FIG. 9 is a schematic representation of an inductive local field monitoring unit employed in the present invention.

FIG. 10 is a schematic representation of a capacitive local field monitoring unit employed in the present invention;

figure 11 is a schematic representation of a magnetic resonance examination system in which the RF transmit module of the present invention is incorporated.

Detailed Description

Fig. 1 is a schematic representation of an RF transmit module of the present invention. A signal lead 111, such as a coaxial cable, couples the RF power supply 15 to the

RF transmission elements

13, 16. The RF power supply is implemented as an RF amplifier, which is typically a regulated amplifier. The RF transmit element is implemented as a tuned coil loop. The RF transmitting element may be a dipole coil element as used for UHF applications. Here the loop is located in the center of the dipole, since the current there is the largest.

The RF transmit elements may also be TEM coil elements. The local field monitoring unit 101 picks up the transmitted RF (B) from the tuned coil loops 13, 16₁) A small amount of energy of the field. The local field monitoring is implemented for example as a small pick-up coil. The low puc signal generated by the pick-up coil is a weak voltage or current signal. The low puc signal is amplified by a preamplifier, which is preferably integrated with the pick-up coil. The amplified puc signal is applied to a mixer 105 for frequency conversion. Preferably, viaThe amplified puc signal is down-converted to a frequency band of tens of MHz, e.g. around 50MHz, e.g. in the range between 10MHz and 100MHz, and coupled into the signal lead 111 by the splitter 107. The oscillator signal for mixer 105 is generated by oscillator 207. The oscillator signal is coupled into signal lead 111 at splitter 109 and coupled out (outcouple) from signal lead 111 to the control port of mixer 105 at splitter 109. At the input port of the mixer, the amplified puc signal is applied. The down-converted puc signal is coupled out from signal lead 111 to RF controller 120 through splitter 109. The main function of the RF controller 120 is to control the RF amplifier 15 to generate a drive signal in accordance with a selected RF waveform. Based on the down-converted puc signal, the RF controller 120 controls the RF amplifier so that the desired B₁The field is emitted by the tuned

coil loops

13, 16. Furthermore, the local field monitoring unit may detect that the electric field of the

RF antenna elements

13, 16 exceeds a predetermined safety level. In this case, based on the puc signal, the RF controller is configured to turn off the RF power supply 15 to avoid a dangerous situation of high SAR from occurring or continuing.

Fig. 2 is a schematic representation of a more detailed embodiment of the RF transmit module of the present invention. The down-converted puc signal is then formed into a frequency difference signal of the amplified puc signal and the oscillator signal. For example, the puc signal is in the 127MHz puc band corresponding to the 3T main field strength of the magnetic resonance examination system. The oscillator signal is for example in the 177MHz oscillator frequency band, so that the down-converted puc signal is in the 50MHz frequency band with low loss on signal lead 111. A stop band notch filter 211 blocking the 177MHz oscillator band is arranged between the mixer and the splitter 107. Another stop band notch filter 213, which blocks the puc band, is electrically connected between mixer 105 and single lead 111. Stop

band notch filters

211 and 213 are arranged as a hybrid-duplexer circuit 210. The mixer-duplexer circuit is operative to provide the oscillator signal only to the control port and not to the output port of the mixer. The mixer-diplexer circuit also serves to prevent the puc signal from bypassing the mixer into signal lead 111. A puc-stop band notch 203 filter having a stop band corresponding to the frequency band of the RF drive signal (127MHz) is electrically connected between the mixer 105 and the splitter 107 to prevent the RF drive signal from reaching the mixer 105.

Another oscillator-stop band notch filter 205 is electrically connected between the oscillator 207 and the splitter 109 to block the RF drive signal from reaching the output port of the oscillator. To avoid the oscillator signal from reaching the RF controller, the stop band notch filter 223 is electrically connected between the output port of the oscillator and the RF controller 120. Another stop band notch filter 221 is electrically connected between the splitters 109 to avoid any residual signal in the puc band from reaching the oscillator. The stop

band notch filters

221 and 223 are implemented as an oscillator-duplexer 220. The down-converted puc signal is fed through an oscillator-duplexer 220 to a band pass filter 209 to an analog-to-digital converter (ADC) 215. The bandpass filter matches the down-converted puc signal to the input characteristics of the ADC. The digital puc signal from the ADC is fed to a digital RF controller 120 to control the RF amplifier 15.

A transmit-receive (T/R) switch 227 is disposed in the signal lead 111 to switch the RF transmit module between transmit and receive functions. In the receive function, the RF signals for the

RF antenna elements

13, 16 are fed to a signal amplifier and then to a reconstructor. In the transmit function, RF power supply 15 generates drive signals on signal leads 11 to activate the RF antenna elements to transmit the B1 field. Matching circuitry 201 is provided to couple the signal leads 111 to the

RF antenna elements

13, 16.

Fig. 3 and 4 show diagrams of matching circuits employed in the present invention. The

RF transmission elements

13, 16 are formed as coil loops with tuning

capacitors

302, 303 to resonate the coil loops in the larmor frequency band for transmission or reception. A tunable capacitor is provided as tuning capacitor 303. The coil loops are coupled to the signal leads 111 by capacitive matching circuitry 201, which includes matching capacitors 301. In the alternative of fig. 4, matching circuitry inductively couples the coil loops to the signal leads through transformer 401, and additional capacitive matching is accomplished by matching capacitors 403.

Fig. 5, 6 and 7 show alternatives to stop band notch filters. The embodiment of fig. 5 is an LC series circuit of an inductor 510 and a capacitor 503. The embodiment of fig. 6 is 1/4 lambda lead 505 tuned to the appropriate frequency stop band. The embodiment of fig. 7 is an LC series circuit of an inductor 517 and a capacitor 517, arranged in series at a common point between parallel LC circuits.

Fig. 8 shows a circuit diagram of an example of a duplexer. The duplexer has two filter branches connected to a common point. The high frequency branch is formed by

series capacitances

601, 603 between which an inductor 609 is connected in circuit fashion. The high frequency branch has a high frequency (e.g., 177MHz) passband. The low frequency branch is formed by

series inductances

605, 607, and a capacitor 611 is connected in circuit between the series capacitances. The low frequency branch has a low frequency (e.g., 50MHz) passband. At the common point, both the high frequency signal and the low frequency signal are present and are split into high frequency and low frequency components on the respective filter branches. This circuit can be used as a mixer-duplexer 210 having at its common point an output down-converted puc signal as a low frequency signal and an oscillator signal as a high frequency signal. The circuit of fig. 8 may also be used as an oscillator-duplexer 220. Then, at the common point, the high frequency signal is the oscillator signal from the oscillator, and the low frequency signal is the down-converted puc signal coupled out from signal lead 111 to ADC 215.

FIG. 9 is a schematic representation of a local inductive field monitoring unit employed in the present invention. The puc coil loops 101 are mounted on portions of the strips of the

coil loops

13, 16 of the

RF antenna elements

13, 16. However, the puc loop coil has no galvanic contact with the strips of the coil loop. The puc coil loop 101 is connected to a preamplifier 103. The output amplified puc signal from the preamplifier is applied to the mixer through a balun 701. The balun suppresses the cable current such that B of the puc coil loop₁The field distribution is not distorted. The balun provides symmetry for the puc coil loop. In this way, the balun avoids the cable shielding of the coaxial cable to distort the puc-coil loop and the symmetry of the puc-coil loop, thereby becoming less efficient.

Fig. 10 is a schematic representation of a capacitive local field monitoring unit employed in the present invention. The capacitors 101 are arranged on the

strips

13, 16 of the coil loop. The local capacitor 101 is connected to a preamplifier 103. The output amplified puc signal from the preamplifier is applied to the mixer through a balun 701.

Figure 11 diagrammatically shows a magnetic resonance imaging system using the present invention. A magnetic resonance imaging system comprises a main magnet with a set of main coils 10, whereby a steady, uniform magnetic field is generated. The main coil is constructed, for example, in such a way that: i.e. they form a bore to surround a tunnel-like examination space. The patient to be examined is placed on a patient carrier which is slid into the tunnel-like examination space. The magnetic resonance imaging system further comprises a plurality of gradient coils 11, 12, whereby a magnetic field exhibiting spatial variations, in particular in the form of temporary gradients in individual directions, is generated so as to be superposed on the homogeneous magnetic field. The gradient coils 11, 12 are connected to a gradient controller 21 comprising one or more gradient amplifiers and controllable power supply units. The gradient coils 11, 12 are energized by applying a current by means of the power supply unit 21; to this end, the power supply unit is fitted with electronic gradient amplification circuits that apply currents to the gradient coils in order to generate gradient pulses (also called "gradient waveforms") of appropriate temporal shape. The strength, direction and duration of the gradient are controlled by the control of the power supply unit. The magnetic resonance imaging system further comprises transmit and receive antennas (coils or coil arrays) 13, 16 for generating the RF excitation pulses and for picking up the magnetic resonance signals, respectively. The transmission coil 13 is preferably constructed as a body coil 13, whereby (parts of) the object to be examined can be surrounded. The body coil is typically arranged in a magnetic resonance imaging system in such a way that: so that a patient to be examined placed on a patient carrier 14 (e.g. a movable patient table) is surrounded by the body coil 13 when he or she is arranged in the magnetic resonance imaging system. The body coil 13 serves as a transmit antenna to transmit the RF excitation pulses and the RF refocusing pulses. Preferably, the body coil 13 involves a spatially uniform intensity distribution of the transmitted RF pulses (RFs). The same coil or antenna is usually used alternately as the transmitting coil and the receiving coil. Typically, the receive coil comprises a plurality of elements, each element typically forming a single loop. Various geometries of the shape of the loop and arrangements of the various elements are possible. The transmission and reception coil 13 is connected to an electronic transmission and reception circuit 15.

It should be noted that there is one (or several) RF antenna element(s) that can be used for both transmission and reception; furthermore, in general, the user may choose to employ a dedicated receive antenna, typically formed as an array of receive elements. For example, the surface coil array 16 may be used as a receive and/or transmit coil. Such surface coil arrays have a high sensitivity in a relatively small volume. The receive coil is connected to a preamplifier 23. The preamplifier 23 amplifies the RF resonance signal (MS) received by the receiving coil 16 and applies the amplified RF resonance signal to the demodulator 24. The receiving antenna (e.g. the surface coil array) is connected to a demodulator 24 and the received pre-amplified magnetic resonance signals (MS) are demodulated by means of the demodulator 24. The preamplifier 23 and the demodulator 24 may be digitally implemented and integrated in the surface coil array. The demodulated magnetic resonance signals (DMS) are applied to a reconstruction unit. The demodulator 24 demodulates the amplified RF resonance signal. The demodulated resonance signal contains the actual information about the local spin densities in the part of the object to be imaged. Further, the transmission and reception circuit 15 is connected to a modulator 22. The modulator 22 and the transmission and receiving circuit 15 activate the transmission coil 13 so as to transmit the RF excitation and refocusing pulses. In particular, the surface receive coil array 16 is coupled to the transmit and receive circuitry by a wireless link. The magnetic resonance signal data received by the surface coil array 16 is transmitted to the transmit and receive circuitry 15 and control signals (e.g., tuning and demodulating surface coils) are transmitted to the surface coils over a wireless link.

The reconstruction unit derives one or more image signals from the demodulated magnetic resonance signals (DMS), which image signals represent image information of the imaged part of the object to be examined. In practice, the reconstruction unit 25 is preferably constructed as a digital image processing unit 25 which is programmed to derive from the demodulated magnetic resonance signals image signals representing image information of the part of the object to be imaged. The signal on the output of the reconstruction is applied to a monitor 26 so that the reconstructed magnetic resonance image can be displayed on the monitor. Alternatively, the signal from the reconstruction unit 25 can be stored in a buffer unit 27 while awaiting further processing or display.

The magnetic resonance imaging system according to the invention is further provided with a control unit 20, for example in the form of a computer comprising a (micro) processor. The control unit 20 controls the operation of the RF excitations and the application of the temporary gradient fields. For this purpose, the control software is loaded, for example, into the control unit 20 and the reconstruction unit 25. The control software supervises the RF controller 120 of the RF transmit module. The control software also supervises the gradient control 21.

Claims

1. A Radio Frequency (RF) transmit module for a magnetic resonance examination system, the RF transmit module comprising:

an RF-transmitting antenna element for transmitting the radio frequency signal,

an RF power supply having an RF control, the RF power supply configured to generate an RF drive signal,

a signal lead electrically connected between the RF power source and the RF transmit antenna element, the signal lead configured to supply the RF drive signal to the RF transmit antenna element,

a local field monitoring unit configured to measure local field strengths transmitted by the RF transmit antenna elements and to generate pick-up coils, puc, signals,

a mixer configured to frequency convert the puc signals to frequency converted puc signals and electrically connected between the local field monitoring unit and the signal lead to transmit the frequency converted puc signals to the RF control through the signal lead.

2. The RF transmit module of claim 1 wherein the frequency converted puc signal has a carrier frequency in the frequency range of 1MHz to 100 MHz.

3. The RF transmit module of claim 1 or 2, further comprising:

a preamplifier for amplifying the puc-signal, wherein the preamplifier is electrically connected between the local field monitoring unit and the mixer, and

wherein the preamplifier is electrically connected to apply a pre-amplified puc signal to the mixer to generate the frequency converted puc signal.

4. The RF transmit module of claim 3 wherein the preamplifier is located near the local field monitoring unit or on a board of the local field monitoring unit.

5. The RF transmit module of claim 1 or 2, comprising:

a plurality of RF transmit antenna elements and a plurality of signal leads coupling the RF transmit antenna elements to the RF power source,

a plurality of local field monitoring units, each of the plurality of local field monitoring units associated with one of the plurality of RF transmit antenna elements to measure a local field strength transmitted by the one of the plurality of RF transmit antenna elements, wherein the mixer is configured to frequency convert puc signals generated by the plurality of local field monitoring units into frequency converted puc signals and to electrically connect between individual local field monitoring units and the signal lead to transmit the frequency converted puc signals to the RF control through the signal lead.

6. The RF transmit module of claim 1 or 2, comprising:

an oscillator coupled to the signal lead and configured to transfer an oscillator signal to the mixer through the signal lead, an

An oscillator notch filter electrically connected between the signal lead and the oscillator and configured to attenuate the RF drive signal.

7. The RF transmit module of claim 1 or 2, comprising:

a matching unit for impedance matching between the RF transmitting antenna element and the signal lead, the matching unit being electrically connected between the signal lead and the RF transmitting antenna element;

a mixer notch filter is electrically connected between the signal lead and the mixer to attenuate the RF drive signal, and wherein,

the matching unit, the mixer notch filter, the local field monitoring unit and the preamplifier are integrated in a single integrated monitoring module, which is directly connected to the RF transmit antenna element.

8. The RF transmit module of claim 6 comprising a mixer-duplexer electrically connected between the signal lead and the mixer and configured to separate a frequency converted puc signal from the mixer from the oscillator signal to the mixer.

9. The RF transmit module of claim 6 comprising an oscillator-duplexer, the oscillator-duplexer electrically connected between the signal lead and the oscillator and configured to separate a frequency-converted puc signal from the signal lead from the oscillator signal.